1. Field of the Invention
The present invention relates to surge arrestors and, more particularly, to gas discharge tube surge arrestors for coaxial transmission lines.
2. Discussion of the Relevant Art
Numerous gas discharge tube surge arrestors have been developed over the years for protecting telephone lines from overvoltage conditions caused, for example, by lightning or fallen high voltage power lines. Such conventional surge arrestors, while suitable for telephone lines, are unsuitable for coaxial transmission lines which have unique characteristics and requirements. Several attempts have, however, been made to provide gas discharge tube surge arrestors for coaxial transmission lines.
Kawanami U.S. Pat. No. 4,544,984 issued Oct. 1, 1985 (Kawanami '984) discloses a gas discharge tube surge arrestor for a coaxial transmission line. According to the Kawanami '984 patent, conventional gas discharge tubes, while suitable as surge arrestors for telephone lines, cannot be used for high frequency coaxial transmission lines because (1) the gas discharge tube has a considerable amount of capacitance and (2) the nature of the required connection is such that it greatly changes the impedance of the coaxial transmission line and causes reflections in the transmission line. According to the Kawanami '984 patent, there has previously been no surge arrestor which could be used in a high frequency coaxial transmission line (column 1, line 57 to column 2, line 4).
The Kawanami '984 patent discloses a surge arrestor which connects a gas discharge tube between the inner and outer conductors of the coaxial transmission line in a direction orthogonal to the direction of signal transmission. The unwanted increased capacitance associated with the use of a gas discharge tube in a coaxial transmission line is compensated for by reducing the effective cross sectional area of the inner conductor at the place where the gas tube contacts the inner conductor by cutting out a portion of the center conductor to create a flat area on which the gas tube rests.
Kawanami U.S. Pat. No. 4,509,090 issued on Apr. 2, 1985 (Kawanami '090) also explains why conventional gas discharge tubes have not been successfully employed as surge arrestors in coaxial transmission lines and discloses the same type of structure disclosed in the Kawanami '984 patent, i.e., a device which connects the gas discharge tube between the inner and outer conductors of the coaxial transmission line in a direction orthogonal to the direction of signal transmission. In FIG. 7 the Kawanami '090 patent provides information concerning the impact of reducing the effective cross sectional area of the center conductor at the place where it contacts the gas discharge tube, showing that small dimensional changes on the order of 1 or 2 millimeters have a significant effect on the voltage standing wave ratio (VSWR).
Mickelson U.S. Pat. No. 4,633,359 issued on Dec. 30, 1986 also discloses a surge arrestor for a coaxial transmission line in which a gas discharge tube is connected between the inner and outer conductors of the transmission line in a direction orthogonal to the direction of signal transmission. The asserted advantage of the Mickelson device is that it is "simpler and less expensive to fabricate." Like the Kawanami '090 and '984 patents, Mickelson uses a center conductor which is flattened at the place where the gas tube contacts the center conductor. In addition to serving as a seat for the gas tube, this flat area adjusts the inductance of the center conductor to compensate for the distributed capacitance of the gas tube. Chamfers are provided adjacent the flat area to match the impedance of the surge arrestor to that of the transmission line. It is well known that maximum power transfer occurs when matched impedances are employed.
The present invention provides a new and improved surge arrestor for coaxial transmission lines in which the axis of the gas discharge tube is oriented parallel to the direction of signal transmission, rather than orthogonal to the direction of signal transmission as disclosed in the prior art, and the RF signal flows through the gas discharge tube. The coaxial surge arrestor of the present invention is sufficiently small that it can be incorporated within or made an integral part of existing coaxial connectors. Further, the present invention results in a much simpler, easier to manufacture and, therefore, less expensive device. At the same time, the present invention permits compensating for the unwanted capacitance introduced by the presence of a gas discharge tube in the coaxial transmission line and further permits matching the impedance of the surge arrestor to that of the coaxial transmission line so as to provide a device having a useful frequency range extending from 50 MHz to at least 1 GHz.
Therefore, it is an object of the present invention to provide a coaxial surge arrestor which has a characteristic impedance similar to that of the coaxial transmission line.
It is another object of the present invention to provide a coaxial surge arrestor which permits compensating for the unwanted capacitance introduced by the use of a gas discharge tube in a coaxial transmission line.
It is another object of the present invention to provide a coaxial surge arrestor which may be mounted within conventional coaxial cable components and which may be readily installed in existing coaxial transmission lines.
It is another object of the present invention to provide a gas discharge tube suitable for use in a coaxial surge arrestor.
It is another object of the present invention to provide a coaxial surge arrestor in which the RF signal flows through the gas discharge tube.
It is another object of the present invention to provide an economically constructed coaxial surge arrestor which includes fail safe protection so that overheating of the gas discharge tube will short the communication line to ground, thereby protecting the equipment to which it is connected.
It is still another object of the present invention to provide a coaxial surge arrestor which includes current limiting and/or low voltage protection.